Implantable medical devices (IMDs) are now being equipped with biosensor systems capable of monitoring optical characteristics (e.g., changes in refractive index or reflectivity) indicative of physiological conditions (e.g., temperature, pressure, blood oxygen content, rate of chemical processing, etc.). An IMD may now be equipped with, for example, a fiber-linked optical interferometric system capable of monitoring hydrostatic pressure at a chosen site within a patient's body; e.g., blood pressure within an artery. In such a system, the proximal end of a flexible, elongated catheter is coupled to an IMD and the distal end of the catheter is positioned adjacent the site to be optically monitored. The catheter carries an optical fiber, which is optically coupled to a transceiver disposed within the IMD's canister. The transceiver directs outgoing light signals into the proximal end of the fiber, which propagate through the optical fiber until they reach the fiber's distal end. The light signals are then modulated by the body fluid (e.g., blood) being monitored and are reflected back into the fiber. The modulated signals propagate through the optical fiber once again and are received by the transceiver at the fiber's proximal end. The transceiver analyzes characteristics (e.g., amplitude in an interferometer) of the returning signals, and control circuitry coupled to the transceiver determines the blood pressure at the distal end of the catheter.
An optical feedthrough is utilized to guide the optical fiber through the canister of the IMD. The feedthrough may comprise a ferrule (e.g., titanium) having an aperture therethrough through which the optical fiber passes. To protect the circuitry of the IMD and to secure the optical fiber within the ferrule, a hermetic seal is formed between an inner surface of the ferrule and an outer surface of the optic fiber. Traditionally, the hermetic seal has typically been formed by way of a co-firing or brazing process. For example, a window-ferrule braze may be formed by threading an annular ceramic or metal (e.g., gold) preform over the window and positioning the preform against an inner shelf provided within the ferrule. If a matched seal is to be formed, the components are chosen to have similar coefficients of thermal expansion, and an inner surface of the ferrule may be metalized prior to insertion of the preform. Next, the feedthrough assembly is heated in a furnace (e.g., to over 700 degrees Celsius for approximately 10-15 minutes) to cause the brazing compound to wet the glass and flow against the ferrule to form a seal. An annealing step is then performed, and the feedthrough assembly is allowed to cool to room temperature.
Glass-to-metal seals are relatively rigid and thus may crack if placed under significant mechanical and thermal stress, which may promote the chemical degradation of the seal. Furthermore, conventional co-firing or brazing processes utilized to produce glass-to-metal and ceramic-to-metal seals subject the optical fiber, or window, to extreme temperatures and thus limit the materials from which the fiber may be made, notably eliminating from consideration plastic optic fibers (POFs) made from flexible and low-cost polymers such as polymethylmethacrylate (PMMA), polystyrene, and polycarbonate. Additionally, co-firing and brazing processes may be relatively complex, costly, and time consuming to perform.
In view of the above, it should be appreciated that it would be desirable to provide an optical feedthrough assembly suitable for use in conjunction with optical fibers comprising a wide range of materials, including POFs. In addition, it would be advantageous if such an optical feedthrough assembly employed a polymeric compression seal that tolerates mechanical stress relatively well and that exhibits a high degree of resistance to chemical degradation. It would be of further benefit if such feedthrough assembly could be manufactured economically and efficiently. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.